A molecular and evolutionary study of the beta-globin gene family of the Australian marsupial Sminthopsis crassicaudata

Beta-globin gene families in eutherians (placental mammals) consist of aset of four or more developmentally regulated genes which are closelylinked and, in general, arranged in the order 5'-embryonic/fetal genes-adult genes-3'. This cluster of genes is proposed to have arisen by tandemduplication of ancestral beta-globin genes, with the first duplicationoccurring 200 to 155 MYBP just prior to a period in mammalian evolutionwhen eutherians and marsupials diverged from a common ancestor. In thispaper we trace the evolutionary history of the beta-globin gene family backto the origins of these mammals by molecular characterization of thebeta-globin gene family of the Australian marsupial Sminthopsiscrassicaudata. Using Southern and restriction analysis of total genomic DNAand bacteriophage clones of beta-like globin genes, we provide evidencethat just two functional beta-like globin genes exist in this marsupial,including one embryonic- expressed gene (S.c-epsilon) and oneadult-expressed gene (S.c-beta), linked in the order 5'-epsilon-beta-3'.The entire DNA sequence of the adult beta-globin gene is reported and shownto be orthologous to the adult beta-globin genes of the North Americanmarsupial Didelphis virginiana and eutherian mammals. These results,together with results from a phylogenetic analysis of mammalian beta-likeglobin genes, confirm the hypothesis that a two-gene cluster, containing anembryonic- and an adult-expressed beta-like globin gene, existed in themost recent common ancester of marsupials and eutherians. Northern analysisof total RNA isolated from embryos and neonatals indicates that a switchfrom embryonic to adult gene expression occurs at the time of birth,coinciding with the transfer of the marsupial from a uterus to a pouchenvironment.